1. Field of the Invention
The invention relates to a method of projecting an image onto a plurality of target areas on a substrate whereby use is made of a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate;
a projection system for imaging an irradiated portion of the mask onto a target area of the substrate,
whereby the substrate is to be irradiated with images from at least two different masks.
2. Discussion of the Related Art
A lithographic projection apparatus as described in the opening paragraph can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire reticle pattern onto the target area at once; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target area is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
Lithographic apparatus may employ various types of projection radiation, such as ultra-violet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d. The apparatus may comprise components which are operated in vacuum, and are correspondingly vacuum-compatible. As mentioned in the previous paragraph, the apparatus may have more than one substrate table and/or mask table.
In many applications of a lithographic projection apparatus, each target area on a given substrate is exposed using a single mask (per layer). However, in certain applications, it is desirable to expose each target area to a patterned image from two or more different masks (both images being projected onto the same layer of radiation-sensitive material on the target area); within each target area, these distinct mask images may, for example, be projected in proximity to one another, in juxtaposition with one another, or in overlap (to a lesser or greater degree). Alternatively, one can expose a layer of radiation sensitive material on a substrate using two or more different masks in such a manner that different mask images are projected onto different target areas, such that each target area will be exposed with one mask image. However, a problem with such multiple-mask exposure is that it incurs a substantial time penalty per substrate. This is because, after exposure of the (relevant) target areas on a substrate using the first mask, the first mask has to be removed from the mask table, has to be replaced by a second mask, and then this second mask has to be aligned with the substrate. Even when employing the fastest and most sophisticated mask handling apparatus, this interchange procedure can be very time consuming. Another problem is that, after each mask change, the optics (e.g. masking shutters) in the radiation system (illuminator) will generally have to be adjusted. Not only does this incur a further time penalty, but it also necessitates much more frequent use of the actuators used to adjust the optics, leading to problems of early wear.
It is an object of the invention to alleviate these problems. In particular, it is an object of the invention to provide a multiple-mask exposure method that allows a significantly greater machine throughput than known methods.
These and other objects are achieved in a method as specified in the opening paragraph, characterized by the following steps:
(a) providing a batch of substrates, each at least partially coated with a layer of radiation-sensitive material;
(b) providing storage means for temporary storage of the batch;
(c) providing a first mask on the mask table;
(d) irradiating a first set of target areas of a first substrate with an image from the first mask, and then placing that substrate in the storage means;
(e) repeating step (d) for each of the other substrates in the batch;
(f) replacing the first mask by a second mask;
(g) providing a primary substrate from the storage means on the substrate table and irradiating a second set of target areas of that substrate with an image from the second mask;
(h) repeating step (g) for each of the other substrates stored in the storage means.
The term xe2x80x9cbatchxe2x80x9d as used in the context of the present invention should be interpreted as referring to a set of substrates that are to be processed according to a given xe2x80x9crecipexe2x80x9d, e.g. in relation to a particular layout or combination of target areas or die types on the available substrate area. Such a xe2x80x9cbatchxe2x80x9d of substrates is offered to the lithographic projection apparatus in one go. In the event that the total number of substrates to be processed according to the said recipe is greater than the size of the said batch, then many such batches may be consecutively offered to the apparatus, until the relevant recipe set is exhausted.
The method according to the invention has the advantage that a mask interchange only has to occur once per batch instead of one per substrate, so that there is less time-overhead per substrate for each such interchange, and so that there is less wear of adjustable optical components in the radiation system. The wear of the masks and the mask handling system will also generally decrease when the method according to the invention is used. The inventors arrived at the invention only after they had discovered that, contrary to what one might think, the radiation-sensitive layer on a given substrate does not degenerate significantly while the substrate is waiting in the storage means between exposure to a first and a second mask, at least not for batches of a reasonable size (e.g. of the order of 25 to 50 substrates). Such degeneration might be expected to occur on the basis of viscous creep, for example.
With regard to step (d), it should be noted that the last substrate to be exposed to the first mask need not be placed in the storage means; instead, it can be left on the substrate table during performance of step (f). In that case, the primary substrate to be exposed in step (g) is already on the substrate table, and thus does not have to be retrieved from the storage means. This situation should be regarded as falling within the scope of claim 1. On the other hand, it may be advantageous to choose the sequence in which substrates are exposed from the second mask so as to be equal to the sequence in which substrates are exposed from the first mask, because, for each substrate in the batch, the delay between both exposures will then be substantially equal, which gives similar processing circumstances for all the substrates. In any case, it should be noted that the primary substrate in step (g) may or may not be the same as the first substrate in step (d): such is a matter of choice and/or requirement in a particular application.
In a particular embodiment of the method according to the invention, the storage means comprise a cassette having slots into which the various substrates can be placed in stacked arrangement. Such cassettes are commonly available for storing and transporting semiconductor substrates (wafers), and are often employed in so-called FOUP or SMIF devices in the industry. An advantage of using such a cassette is that it allows storage of a relatively large number of substrates in a relatively compact area.
In a particular embodiment of the invention, the storage means are located within the lithographic projection apparatus, in proximity (e.g. within the order of 0.5-1.0 meters) to the substrate table and a substrate handler that can be used for transferring each substrate between said substrate table and said storage means. An advantage of such an arrangement is that the time required to load/unload and transfer substrates between the substrate table and the storage means is minimal, with the attendant advantageous effect on batch processing time.
The batch of substrates may be provided to the substrate table with the aid of a wafer track, for example, which spin-coats the substrates with radiation-sensitive material before they are transferred into the lithographic projection apparatus. In a particular embodiment of such a wafer track, the storage means are provided within the track, and the track is equipped with transport means that can be used for transferring a substrate between the substrate handler (of the lithographic projection apparatus) and the said storage means. The batch of resist-coated substrates may also be provided in a cassette and loaded via a substrate entry port; such a cassette can, if desired, double up as the storage means, or separate storage means can be employed.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.